[PATCH] docbook: Tell users to install xmlto, not stylesheets

We have at least two users which were confused by these messages, myself
included.

Signed-off-by: Alexey Dobriyan <adobriyan@mail.ru>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>